1. Field of the Invention
The present invention relates to a digital video signal recording and reproducing apparatus featuring high efficiency coding of digital video signals.
2. Description of the Prior Art
To compose a video signal recording and reproducing apparatus (a VCR, video disc machine or the like), band compression technology (or high-efficiency coding technology) for reducing the amount of data of the original video signals is used for long-time recording.
As one example of such band compression technology, the orthogonal transformation coding employing a Hadamard transformation of a discrete cosine transformation (DCT) is known. In the orthogonal transformation coding, first the video signal is divided into blocks of small regions of, for example, 8.times.8 continuous pixels consisting of 8 horizontal pixels and 8 vertical pixels. Then each frequency component obtained by frequency separation in block units is coded. At this time, as for the high frequency components which have a small effect in the deterioration of the visual characteristics, the amount of data of the original video signal is curtailed by a technique of reducing the data allotment quantity or other method, and it is none of the known effective band compressing techniques noted for a small deterioration in the picture quality. Also known is a technique of allotting variable length codes differing in the code length depending on the value of the data obtained orthogonal transformation. In this method, depending on the frequency of occurrence of the value of the data, the code word of a shorter code length is allotted to the value of the higher frequency of occurrence, and the amount of data is reduced on the whole.
As a conventional example, the video signal recording and reproducing method of a VCR is explained below.
The data curtailed by the orthogonal transformation coding which is one of the known band compression techniques is recorded on a tape by variable length coding, and at this time as the minimum synchronizing unit in reproduction, it is recorded by dividing the data into sync blocks of a data unit having a specific amount of data. A sync block is composed of a synchronizing pattern (SYNC), identification information such as block number and frame number of sync block (ID), video signal coding data, error correction code parity, and other portions.
The amount of data after such variable length coding varies with the input video signal, but it is desirable to keep the tape consumption per unit time constant from the viewpoint of editing and recording time. It is accordingly necessary to control the amount of coding so that the amount of data is constant in a certain range. Furthermore, corresponding to the error propagation, the certain range should be as short as possible. If, however, the certain range is too short, the data occurrence frequency in the certain range fluctuates, and the compression effect by variable length coding is decreased, and the picture quality may deteriorate.
Therefore, in the method of recording and reproducing in sync block units, in the video signal divided into blocks for orthogonal transformation, a specific number of blocks are grouped as a compression unit, and the amount of data is controlled so that this compression unit may have a constant amount of data. The specific amount of data in the compression unit is allotted equally over several sync blocks and recorded.
In such a conventional video signal recording and reproducing apparatus, if it is impossible to detect and reproduce by the data in 1 sync block owing to an error occurring in the process of reproducing, the error propagation for that error spreads over all of the blocks in the compression unit. In such a case, concealment by replacing the data with the data at the same position in the preceding and succeeding screens which are adjacent in time must be effected for all of the blocks within the compression unit. In particular, during high-speed search which is one of the special reproducing techniques, such errors are likely to occur, and concealment is required very often, and if the entire compression units are always concealed, the screen is hardly updated and the search function is spoiled.
As another problem, the amount of information in the compression unit is tremendous, and it is sometimes impossible to settle within a specific amount of data (this phenomenon is called overflow) only by the quantizer incorporated in the apparatus for curtailing the amount of data. In the conventional video signal recording and reproducing apparatus, in such a case, coding data exceeding the specific amount of data cannot be recorded. If the entire data of a certain block is contained in such unrecordable coding data, such an unrecordable block is present even in ordinary recording, and the entire block cannot be reproduced at the time of reproducing.